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    <title>Document</title>
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    <button>点击</button>
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<script>
    var that
    var _that
    class Father {
        constructor(uname){
            that = this
            this.uname = uname
            this.btn = document.querySelector('button')
            this.btn.onclick = this.sing
        }
        sing() {
            console.log(that.uname);
        }
        play() {
            _that = this
            console.log(this);
        }

    }
    var son = new Father('smx')
    son.sing()
    son.play()
</script>
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